1. Field of the Invention
The present invention relates, in general, to a solid state dye-sensitized solar cell employing a composite polymer electrolyte and, more particularly, to a solid state dye-sensitized solar cell employing a composite polymer electrolyte, which includes a photoelectrode, a counter electrode, and an electrolyte sandwiched between the photoelectrode and counter electrode, and in which the composite polymer electrolyte includes one selected from the group consisting of middle molecular substances, or the mixtures of the middle molecular substances and polymer, and the mixtures of the middle molecular substances and inorganic nanoparticles, and redox couples. Here the photoelectrode includes transparent conducting layer, semi-conducting nanoporous layer and dye photosensitizer.
2. Description of the Prior Art
A solar cell, which is capable of generating electricity without emitting a pollutant, thereby providing noteworthy solutions for the protection of environment and energy problems, is being watched with interest due to the exhaustion of fossil fuels and policies restricting carbon dioxide emissions.
FIG. 1 illustrates operation of a conventional dye-sensitized solar cell. As shown in FIG. 1, when sunlight is irradiated to an n-type nanoparticle semiconductor oxide electrode 11 which includes dye molecules (not shown) chemically adsorbed onto a surface thereof, an electronic transition of the dye molecules from a ground state (D+/D) into an excited state (D+/D*) is initiated to form a pair of electron holes, and electrons in the excited state are introduced into a conduction band (CB) of semiconductor nanoparticles. The electrons, introduced into the semiconductor oxide electrode 11, are transferred through interfaces between the particles into a transparent conducting oxide (TCO) 12 which is in contact with the semiconductor oxide electrode 11, and then moved through an external wire 13, connected to the transparent conducting oxide 12, to a counter electrode 14. A redox electrolyte 15 is introduced between the counter electrode 14 and the semiconductor oxide electrode 11, and a load is connected to the transparent conducting oxide 12 and counter electrode 14 in series to measure a short-circuit current, an open-circuit voltage, and a fill factor, thereby evaluating efficiency of the solar cell.
The dye molecules (D+D*), which are oxidized due to the electronic transition caused by the light absorption, receive electrons (e−), generated by oxidation of iodine ions (I3−/I−), in the redox electrolyte to be reduced, and I− ions are reduced by electrons (e−), reaching the counter electrode, thereby completing the operation of the dye-sensitized solar cell. A photocurrent is caused by diffusion of the electrons introduced into the semiconductor electrode, and a photovoltage (Voc) is determined by a difference between Fermi energy (EF) of the semiconductor oxide and a redox potential of the electrolyte.
A representative example of conventional dye-sensitized solar cells is a solar cell known in 1991 by Grätzel et al. in Switzerland (U.S. Pat. Nos. 4,927,721 and 5,350,644). The solar cell suggested by Grätzel et al. is a photo-electrochemical solar cell employing an oxide semiconductor, which includes photosensitive dye molecules and titanium dioxide nanoparticles, and has an advantage of lower production costs than a conventional silicone solar cell. A conventional dye-sensitized nanoparticle oxide solar cell includes a nanoparticle oxide semiconductor cathode, a platinum anode, a dye coated on the cathode, and a redox liquid electrolyte employing an organic solvent. However, when an external temperature of the dye-sensitized solar cell, including the liquid electrolyte produced using the organic solvent, is increased due to sunlight, the solvent of the electrolyte is likely to become volatilized. Accordingly, use of the solar cell in long-term stable and commercial applications is very unsuitable because of a solvent leak.
An initial effort was made to develop a solar cell, which employs a solvent-free solid polymer electrolyte in 2001 by De Paoli et al. in Brazil (A. F. Nogueira, J. R. Durrant, M. A. De Paoli, Adv. Mater. 13, 826, 2001). They have created a polymer electrolyte, which includes a copolymer of poly(epichlorohydrin) and ethylene oxide, and a redox derivative (NaI/I2). Energy conversion efficiency of the solar cell is 2.6% at 10 mW/cm2 and 1.6% at 100 mW/cm2.
Another example is a study, which was conducted in 2002 by Falars et al. in Greece (T. Stergiopoulos, I. M. Arabatiz, G. Katsaros, P. Falars, Nano Letters 2, 1259, 2002), in which semiconductor nanoparticles are added to polyethylene oxide, having high crystallinity, to reduce the crystallinity of the polymer and to improve mobilities of redox moieties. In this regard, significant reduction of a crystal of polyethylene oxide is confirmed using a differential scanning calorimetry (DSC) and an atomic force microscopy (AFM). Hence, ionic conductivity is increased to 10−5 S/cm at room temperature. Additionally, a solar cell has an open-circuit voltage (Voc) of 0.664 V, a short-circuit current (Jsc) of 7.2 mA/cm2, a fill factor of 0.58, and energy conversion efficiency (η) of 4.2% at 65.6 mW/cm2.
However, the solar cell employing the polymer electrolyte is disadvantageous in that its commercialization is impossible because of poor energy conversion efficiency. Other disadvantages are that its ionic conductivity is poorer than that of the solar cell employing the liquid electrolyte (wet solar cell), when a molecular chain of the polymer is long it is difficult to penetrate the electrolyte into pores between the semiconductor nanoparticles, and current density of the solar cell is significantly reduced if the polymer electrolyte insufficiently wraps the semiconductor nanoparticles or is not connected to the semiconductor nanoparticles without a short circuit. Furthermore, in the case of using a liquid-state or wax-state polymer electrolyte having a short molecular chain, mechanical properties of the solar cell are reduced, and an electrolyte leak occurs like the wet solar cell.